(CNN) -- Mars rocks robotically examined by the rover Opportunity were formed at the bottom of a salty sea, NASA scientists said Tuesday.

In a news conference at NASA's Washington headquarters titled "Opportunity hits the Beach," scientists said they don't know how long the water covered parts of the Martian surface or how many years ago it existed.

"Was the water up to your neck or to your ankles? There's no good answer yet," said Steve Squyres, principal science investigator for Opportunity and its twin rover, Spirit.

"There is still a lot we don't know," he said. "We don't know how extensive it was, how long it was there or how common it was on other parts of Mars."

Three weeks ago, NASA presented the first direct evidence that rocks the rovers found on Mars had been shaped by water. But as new clues emerged, NASA scientists revised their theories about the amount of water, said Ed Weiler, NASA associate administrator for space science. And to make certain they were seeing what they thought they were seeing, the NASA scientists sought the experience of a half-dozen geological experts who had nothing to do with the space project.

Those peer reviewers reached the same conclusion: It's reasonable to interpret that the rocks at Meridiani Planum, Opportunity's landing spot, were formed in water, perhaps a shallow, salty sea.

"I was astonished for a couple of reasons," said David Rubin of the U.S. Geological Survey, one of the outside experts consulted by NASA. "On Mars the sedimentary structures look just like on Earth in a beach or a creek."

Geologists looked for specific patterns to determine if layers were worn away by wind or by water. From experiments on Earth, Rubin said, ripples moving in water, probably gentle flows of about a mile an hour, could have created the patterns in the rocks.

"Ripples that formed in wind look different than ripples formed in water," said John Grotzinger, a rover science team member from the Massachusetts Institute of Technology.

The rover Opportunity has found evidence of both wind and water erosion, the scientists said.

The water evidence shows up on the rocks as smile-shaped curves produced by sediments shifting under a current of water. Additional chemical evidence found in the Mars rocks strengthens the belief that standing water was once on the planet.

Three weeks ago the rover scientists delivered evidence of chlorine and bromine in the rocks, an indication that they had soaked in water that was filled with minerals.

The scientists say these findings have profound implications for astrobiology. And it's a good indication that if any micro-fossils exist on Mars, Meridiani Planum would be a good place to go to find them.

"These rocks are good for preserving microbial life," Squyres said. "They are marvelous at preserving micro scales of biological material.

Opportunity will soon be sent across a plain toward a group of rocks in the wall of a crater to gather additional clues to the planet's past.

Spirit and Opportunity have returned more than 18,000 photos and 250,000 measurements from the Martian surface. Both rovers may have time and power to collect even more pictures and data than outlined in the 90 Mars days (92 Earth days) of their primary mission. Spirit landed January 4; Opportunity has been on Mars since January 24.

"We think we may be talking summer, or even September if the batteries hold out," Weiler said. "I think the fun has just begun, frankly."

The next big hurdle for missions to come is to get samples back to Earth, where scientific tests could reveal more insights about the red planet.